Printing apparatus

ABSTRACT

A recording medium is printed upon by exciting it, developing the medium in a toning liquid, and drying the medium. The toning liquid is applied by passing the medium longitudinally around an arc and pumping the toning liquid to flow so that its surface touches the medium on only one face. The latter preferably is substantially liquid impenetrable.

United States Patent 1151 3,701,337

Borelli et al. 1 Oct. 31, 1972 [54] PRINTING APPARATUS 3,094,914 6/ 1963Smith ..95/89 3,416,493 12/1968 Robinson et a1 ..1 18/637 [72] 5 3 igf g3,129,115 4/1964 Clark et a! ..1 18/637 I 3,242,902 3/1966 Ulary..118/637 [73] Assignee: Honeywell, Inc., Minneapolis, Minn. FOREIGNPATENTS 0R APPLICATIONS Dec. France X [21] Appl. No.: 888,769 Pn'mai yExaminer-Mervin Stein Assistant ExaminerLeo Millstein 52 1.1.3.01...118/637, 101/1310. 13,117/37LE, Jam! and Re'lmg ll8/DlG. 23

57 ABSTRA [51] 161.11 ..G03gl3/00 l Cr i 581 Field of Search ..118/637,637 LX, DIG. 23; A rewrdmg medium 15 Pnmed p y s 117/37 x; 101/1316 13developing the medium in a toning liquid, and drying the medium. Thetoning liquid is applied by passing "n as the medium longitudinallyaround an arc and pumping [S6] me (mad the toning liquid to flow so thatits surface touches the UNITED STATES PATENTS medium on only one face.The latter preferably is subl 3,613,701 10/1971 Ando ..134/64 ympenwable 3,169,887 2/ 1965 York ..118/637 14 Claims, 4 Drawing figuresDRIVE CONTROL 28 85 6 i Ga so 64 ea PITENTED 3.701.337

DRIVE CONTROL 1 Fig. 2.

INYEYYORS RONALD F BORELLI Fig 4. PZLD .GARAND PRINTING APPARATUSBACKGROUND OF THE INVENTION This invention relates to methods and meansfor printing upon a recording medium and particularly to methods andmeans for printing permanent images electrographically upon a papermedium at high speeds.

Such high speed printing is desirable in environments such as a computerprint-out. Generally electrographic printing, is accomplished upon anelectrographic paper medium composed of a conductively treated paperbase that supports a plastic dielectric coating. The paper medium ispositioned between an electrode that contacts the conductive base and aand a second electrode whose surface conforms to the shapes to beprinted. A high voltage applied between the two electrodes excites thepaper medium and establishes an electrostatic field across thedielectric coating. The coating retains a residual electrostatic fieldthat constitutes a charged latent image of the shapes to be printed.

The latent image is developed by subjecting the paper medium to a toningliquid composed of charged resin particles which are softened by aliquid carrier. The residual electrostatic field at the dielectricsurface attracts these particles and holds them. This makes the imagevisible. The image is then fixed, i.e., made permanent, by drying toremoving the liquid carrier. This solidifies the particles and bondsthem to the paper. Such drying may be done by heating.

The toning liquid used to develop the latent image consist of thesecharged toning particles and the solvent liquid carrier. Normally, inthe process, large amounts of this carrier liquid are left on and in thepaper medium. This liquid is volatile and when the paper is dried forfixing the developed image large amounts of fumes are expelled into thelocal environment. Large amounts of such fumes are unpleasant andpossibly harmful. Attempts to reduce these fumes to acceptable levels byreducing the amount of liquid carrier used to hold the particles havenot met with success. Poor images resulted. Moreover, large amounts of$UMMARY OF THE INVENTION The invention obviates these deficiencies.According to a feature of the invention only one face of the medium,preferably the dielectric face, is covered with toning liquid. Accordingto another feature this is done by guiding the medium through alongitudinally curved path, and into a controlled flow of toning liquidthat encounters only one face of the medium. Preferably the flow isformed by liquid moving means that discharge a continuous downwardlaminar flow of the liquid over an inclined surface near the path.Drying means then need eliminate only this liquid. Since only a plasticdielectric face need be wetted by the liquid and liquid is not absorbedby a paper base the amount of volatile fumes are created by drying aresubstantially reduced.

According to another feature of the invention the flow is tangential tothe curved path.

According to another feature of the invention the inclined surface formsa gap with the guide means at the curved path. The size of the gap issufficiently small so that when the medium moves through the gap, andthe liquid flows through the gap, the liquid forms a continuous meniscusover the portion of the curved path closest to the inclined surface.

According to another feature of the invention, control means control theflow of toning liquid over the inclined surface. Preferably, thesecontrol means respond to driving means that transport the recordingmedium.

According to yet another feature of the invention, valve means controlthe flow of the liquid from one of several reservoirs carrying differentcolors of the liquid so as to pass one color of liquid over the inclinedsurface.

According to still another feature of the invention the guide meansinclude a roller extending transversely to the longitudinal direction ofthe movement of the medium and the driving means apply a longitudinalforce on the medium so that there exist a continuous normal force overthe are against the roller. The normal force assures uniformdistribution of the flowing liquid. It also prevents any liquid fromentering the side of the medium contacting the roller.

According to still another feature of the invention, the gap issufficiently small so the carrier medium contacts the liquid, butsufficiently large to keep the liquid in contact with only one side ofthe medium.

According to another feature the guide means and inclined surface aremade of conductive materials. These align the residual electrostaticfield lines, that form the image on the medium. Preferably, the gap issmall enough to make these lines perpendicular to the opposing faces ofthe guide means and the surface.

According to still another feature of the invention, the liquid movingmeans include nozzle means for spreading the liquid over the inclinedsurface to distribute it so as to encounter the entire transversedimension of the medium at the meniscus in the gap.

According to still another feature of the invention, the gap size issuch and the flow such that the moving medium shears a portion of theliquid off the meniscus and the flow replaces the portion to retain themeniscus. Preferably, the flow and movement of the medium are in thesame direction.

These and other features of the invention are pointed out in the claims.Other objects and advantages of the invention will become obvious fromthe following detailed description when read in light of the followingdrawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of aprinting system embodying features of the invention;

FIG. 2 is a cross-sectional view of the recording medium in FIG. 1;

FIG. 3 is a sectional view of the medium passing through the meniscus inFIG. 1; and

FIG. 4 is a cross-sectional view of the medium passing through thedrying rolls of FIG. 1.

3 DESCRIPTION OF PREFERRED EMBODIMENT In FIG. 1, a pair of peripherallyengaging elastomeric capstan rolls l and 12 rotating as shown pinch aweb of electrographic paper 14 and draw it from a supply roll 16 uponwhich the paper is wound. A cross section of the paper 14 appears inFIG. 2. The paper is composed of a conductively treated paper-fibrousbase 18 which supports a plastic dielectric coating 20. The paper isrolled on the roll 16 so as to place the coating 20 on the inside. Thepaper 14 constitutes the recording medium. It is referred to as paperalthough only a portion is composed of paper fibers. The base 18 may forexample be .003 inch thick and the coating .0005 inch thick.

Braking means 17 places the paper 14 under longitudinal tension as thecapstan rolls l0 and 12 pull it. This keeps the paper taut. The bearingsin roll 16 may exhibit sufficient friction to constitute the brakingmeans 17.

The paper 14 leaving the supply roll 16 passes over an idler roll 22with the dielectric coating 20 contacting the roll. An excitationstation 23 then excites the paper 14. In the excitation station 23 thepaper 14 passes between the peripheral surface of an idling image roll24 which contacts the coating 20 and an electrode 26 which is biased tofrictionally contact the conductive base 18. The periphery of the roll24 maybe etched so as to project outwardly to form the pattern ofwriting which is to be printed on the paper. It may also be composed ofvariable projecting types that are controlled by a computer to produce acomputer print-out. Essentially the writing makes the image roll 24 aprint roll.

. A 700 volt direct current source 28 applies an excitation voltagebetween the electrode 26 and the image roll 24 which is grounded. Thisforms an electrostatic field across the coating 20 at those portions ofthe periphery on the roll 24 that project and contact the layer 20 andthe conductive base 18. The image roll 24 rotates as the paper 14 passesbetween it and the electrode 26. The roll 24 and the electrode 26 areslightly less wide than the width of the paper 14. The coating 20retains across it a residual electrostatic field corresponding to theexciting field and the image on the periphery of the image roll 24. Thisconstitutes a latent image.

The paper 14 emerging from between the roll 24 and electrode 26 passesover an exit idler roll 30 and around a steel toning roll 32 of adeveloping or toning station 34. The roll 32 guides the paper and ispreferably as narrow or slightly narrower than the paper. The plasticcoating 20 of the paper 14 faces outwardly of the toning roll 32. Sincethe capstan rolls l0 and 12 draw the paper 14 against the restrainingforce at the supply roll 16, the longitudinal forces on the paperproduce radial forces directed inwardly to the axis of the roll 32. Thisassures substantially firm contact between the roll 32 and the base 18of the paper 14.

During this process the dielectric coating 20 of the paper 14 continuesto retain across its thickness the latent electrostatic field thatconstitutes the latent image of the projections on the periphery of theimage roll 24. In order to develop this latent image, a pump 36 drives atoning liquid. The latter comprises a high resistance carrier such askerosene, which has a resistance of 10" ohms per cubic centimeter, and asuspension of charged colored thennoplastic resin particles which formthe printing material. The pump 36 drives this fluid 38 through aflexible pipe 40 to a fan nozzle 42. The latter spreads the fluidlaterally over a flat steel inclined plate 44 to form a shallow laminarliquid flow. The plate is as narrow or narrower than the width of thepaper. The plate 44 is spaced from the freely turning toning roll 32 soas to form a gap 46 between the coating 20 and the plate 44 of from .005to .020 inch. When the liquid 38 flows through the gap 46. it contactsthe coating 20 and forms a meniscus 48. The meniscus appears moreparticularly in FIG. 3 which shows an enlarged cross section of the roll32, the paper 14, the plate 44, the meniscus 48 and the liquid 38 inFIG. I.

As stated the toning liquid 38 flows by gravity to the toning area whereit contacts the paper 14 due to the closeness of the roller 32 and theflat plate 44. As the liquid 38 contacts the paper 14, the meniscus 48is formed at the coating 20. The meniscus is formed and kept intact bythe forces of adhesion between the molecules of the flowing liquid 38and the coating 20 of the paper 14. It extends transversely across theroll 32 and clings peripherally from one side of the gap, at itsnarrowest portion, to the other. The gap 46 is sufficiently small toform such a meniscus but large enough to allow some passage of toningliquid 38.

The thermoplastic resin particles suspended in the kerosene of theliquid 38 are charged and adhere to the portions of the surface 20 whichare oppositely charged. This makes the image visible. As the paper 14 isdrawn away from the capstan rolls l0 and 12 it shea'rs off the topportion of the meniscus 48 and retains a portion of the liquid and thethermoplastic resin particles. The flow that supports the meniscus ismaintained by the constant pumping action of the pump 36. The flow isadjusted so the paper 14 just contacts or kisses" the liquid. This helpsassure even distribution of the liquid and a distribution of theparticles that depends almost exclusively on the electrostatic field. Auniformly developed image results.

The particles continue to adhere to the now wet surface of the coating20. A wiper 50in the shape of a longitudinally notched rod removes someof the liquid 38 on the dielectric coating. The remaining liq uid 38flowing out of the meniscus 48 is caught in a trough 52 and fed back toa selected one of three reservoirs 54, 56 and 58 containing toner liquidof different colors.

The liquid 38 in each reservoir 54, 56 and 58 has suspended thereinthermoplastic resin particles of different colors so as to print indifferent colors. Six pairs of electrically actuated flow valves 60, 62,64, 66, 68, and select which of the fluids 38 are drawn by the pump 36from their respective reservoirs. The valves 60-70 are arranged so thattwo pairs are closed and one pair open to fluid flow.

The tightness of the paper about the roll 32 helps assure uniformdistribution of liquid over the coating 20. It also keeps liquid 38 fromthe meniscus 48 from entering between the roll 32 and base 18 of thepaper 14. Such entry would cause the base to absorb a good deal of theliquid and make drying difficult. The particles in the liquid on thecoating are concentrated in those areas carrying the electrostaticfield.

The steel roll 32 and the steel plate 44 are in close proximity acrossthe gap. In the vicinity of the gap they thus tend to align the residualelectrostatic field across the dielectric 20 to be substantiallyperpendicular to the dielectric. Such alignment prevents fringing. Itassures a sharp visible image. it prevents fuzziness that fringing mayintroduce. The smaller the gap the sharper the image.

An idler roll 70 directs the paper into a drying apparatus 71 where theimage is fixed. In this drying apparatus the paper passes between anelastomeric backup roll 74 that contacts the base of the paper 14 and asecond elastomeric absorption roll 76 that contacts the wettoner-carrying coating 20 of the paper 14. Both of the rolls idle andmove with paper 14. Mountings 77 that support rolls 74 and 76 bias therolls to apply pressure against the paper 14.

As the paper passes through the rolls 74 and 76 compress not only thepaper but a portion of the peripheral surface of the elastomericmaterial. The elastomeric material of the absorption roll 76 is such asto be absorbent of the liquid 38. As the roll 76 has its peripherycompressed near the engagement point with the roll 74, any air in theabsorptive surface or any other fluid in the absorptive surface issqueezed out. As the paper passes through and emerges between the pinchof the rolls, the portion of the roll 76 that has been compressed beginsto expand and absorb the kerosene carrier in the liquid 38 on thesurface of the coating 20 of the paper 14. Those particles which areattracted by their electrophoretic condition to the charged portions ofthe coating 20 remain on and in the crevices of the coating 20.

The concentration of charged particles near the charged portions of thecoating 20 develops the printing to create the visible image. Theremoval of the surrounding liquid 38 produces printing upon the paper 14and fixes the paper. However, the concentration when squeezed may alsotransfer a portion of the image onto the roll 76. This may then betransferred to a successive portion of the moving paper. To preventthis, a rotating cleaning brush 78 that dips into a cleaning fluid 80 ofa trough 82 applies the cleaning fluid to the surface of the elastomerroller 76. This removes any image.

A scraper 84 compresses the surface of the roll 76 after its contactwith the brush 78. The scraper 84 squeezes the surface of theelastomeric roll between itself and the axis of the roll. This squeezesout any toning liquid or cleaning fluid that may exist in the absorptive pores of the roll 76. At the same time it scrapes the toning liquidand cleaning fluid that was on the surface of the roll and that whichwas squeezed out of the roll. This substantially dries the paper 14. Thepaper then passes between the capstan rolls and 12 and out to autilizing station. The scraper avoids the effect of a squeeze rollerwhich would, as it turns, reapply the liquid squeezed out.

A drive control 85 that senses the speed of the capstan rolls 10controls the flow of the pump 36 so that more or less fluid 38 can passout the nozzle 42 in dependence upon the speed of the paper 14. In thatmanner, if the paper speed 14 at the gap 46 is so fast as to shear 011'large amounts of liquid in the meniscus 48 in a short period of time,sufficient fluid is provided to replace the liquid in the meniscus. Thispermits a variable speed printer that can achieve high speeds. The highspeeds are rendered possible because large amounts of liquid can beconcentrated along a thin line across the moving paper. The line can beconstantly replenished.

By virtue of the invention a controlled amount of toning liquid 38 issupplied to only one surface of the paper 14 at any one time. The amountof liquid applied can be controlled by controlling the flow down theplate 44 on the basis of the speed with which the paper moves and shearsoff the liquid. Moreover, because the roll 32 is round and because thecapstan rolls 10 and 12 and the braking means 17 place a longitudinalforce on the carrier, a normal force exist throughout the are over whichthe paper contacts the guide roll 32. This force causes the paper to hugthe guide roll 32 across the arc. Thus, even distribution of liquid isencouraged. Also, liquid is discouraged from entering the portionbetween the toning roll 32 and the dielectric 20 and wetting thefiberous base 18 of the paper 14. This limits the absorption of liquid.It prevents needless vapors.

The steel roll 32 and plate 44 separated by a narrow gap 46 assure asharp developed image by aligning the electrostatic field linesperpendicularly to the surface of dielectric 20 at the gap 46.

It should be noted that the thickness of the paper 14 is somewhatexaggerated for clarity throughout the drawings.

The diameter of the roll 32 is between 1% inch and 3 inches andpreferably 1 inch. Other sizes may of course be used.

While an embodiment of the invention has been described in detail itwill be obvious to those skilled in the art that the invention may beotherwise embodied within its spirit and scope.

What is claimed is:

1. Apparatus for printing on a flat flexible recording medium havingopposing faces, comprising:

A. driving means for moving the medium along a given path;

B. excitation means in the path of the recording medium for exciting themedium according to the image to be printed;

C. developing means in the path of the recording medium for applying atoning liquid to one face of the excited recording medium; and

D. fixing means in the path of the recording medium after saiddeveloping means for fixing the image and removing excess liquid fromthe recording medium;

E. said developing means including:

a. guide means for longitudinally guiding the moving recording mediumalong a continuously curved course such that the medium intersects themoving liquid at only one of its surfaces, and

b. flow producing means for forming a laminar flow of toning liquidhaving a substantially flat surface, said flow producing means includingliquid slide means for forming an inclined upper surface spaced fromsaid guide means so as to form a gap between the curved course and theslide means, and liquid moving means including a flat nozzle means forspreading the liquid over the surface of the slide means so as to makethe liquid flow gravitationally through the gap, the

size of the gap being sufficiently small so that when the recordingmedium is moved through the gap the liquid forms a continuous meniscusat the medium gap, the liquid encountering a continuous band across themedium at the meniscus.

2. An apparatus as in claim 1 wherein A. said flow producing meansincludes control means for controlling the flow of toning liquid.

3. An apparatus as in claim 1 wherein A. said flow producing meansincludes control means responsive to said driving means for controllingthe flow of liquid over said surface.

4. An apparatus as in claim 1 wherein A. said liquid moving meansincludes a plurality of reservoirs of liquid of different colors andmeans for controlling the flow of said liquids so as to selectively passone color of liquid over said slide means.

5. An apparatus as in claim 1 wherein A. said driving means move themedium longitudinally;

B. said guide means include a roller extending transversely to thelongitudinal direction of the movement of the flexible recording mediumand cooperating with said drive means to hole said recording medium tautover an arc of said roller and thereby to apply a normal forcecontinuously over said arc with said recording medium.

6. An apparatus as in claim 1 wherein A. said driving means include acapstan for driving said recording medium in a continuous longitudinalpath through said excitation means, said developing means and saiddrying means, and

B. said driving means further include braking means for restrainingmovement of the recording medium so as to hold it taut.

7. An apparatus as in claim 4 wherein A. said driving means move saidrecording medium along a continuous longitudinal path through saidsensitizing means, said developing means, and said drying means and sothe recording medium extends simultaneously through said sensitizingmeans, said developing means and said drying means.

8. An apparatus as in claim 1 wherein A. the gap between said guidemeans and said slide means is sufficiently small so the recording mediumcontacts the liquid and wherein B. said gap is sufficiently large so asto keep the liquid in contact with only one side of the recordingmedium.

9. An apparatus as in claim 1 wherein A. trough means for collectingliquid passing down said slide means,

B. reservoir means for receiving liquid from said trough means, and

C. pump means for pumping the liquid to the upper surface of said slidemeans;

D. whereby said liquid flows down said slide means.

10. Apparatus for printing on a flat flexible recording medium havingopposing faces comprising;

A. driving means for moving the medium along a given path;

B. excitation means in the path of the recording medium for exciting themedium according to the image to be printed;

C. developing means in the path of the recording medium for applying atoning liquid to one face of the excited recording medium;

D. fixing means in the path of the recording medium after saiddeveloping means for fixing the image and removing excess liquid fromthe recording medium;

B. said developing means including:

a. flow producing means for forming a laminar flow of toning liquidhaving a substantially flat surface, and

b. guide means for longitudinally guiding the moving recording mediumalong a continuously curved course that intersects the moving liquid atonly one of its surfaces; and

F. said liquid removing means including a. first and second engagingmeans for engaging opposite faces of the recording medium between them,

. at least one of said engaging means having an externally curvedsurface so as to permit rolling engagement between said engagement meanswhen the recording medium is moved between them,

c. said first engaging means having a compressible porous absorbentsurface and said second engaging means having a surface,

d. pressure means for pressing the moving recording medium between saidengagement means so that as said recording medium advanceslongitudinally said engagement means squeeze continuously successiveportions of the recording medium and the absorbent surface and thenrelease the recording medium and the absorbent surface so that saidabsorbent surface absorbs liquid on said recording medium,

e. and compression means for compressing the previously squeezed portionof the absorbent surface and scraping it so as to remove the liquidtherefrom and scrape it away.

11. An apparatus as in claim 10 wherein said engagement means eachinclude rotatable roll means adapted to engage opposite faces of thesame portion of the recording medium and squeeze the recording mediumbetween them.

12. Apparatus for printing on a flat flexible recording medium havingopposing faces comprising;

A. driving means for moving the medium along a given path;

B. excitation means in the path of the recording medium for exciting themedium according to the image to be printed;

C. developing means in the path of the recording medium for applying atoning liquid to one face of the excited recording medium;

D. fixing means in the path of the recording medium afier saiddeveloping means for fixing the image and removing excess liquid fromthe recording medium;

E. said developing means including a. flow producing means for forming alaminar flow of toning liquid having a substantially flat surface; and

b. guide means for longitudinally guiding the moving recording mediumalong a continuously curved course that intersects the moving liquid atonly one of its surfaces; and

F. said fixing means including a pair of elastomeric rolls havingabsorptive surfaces for removing excess liquid,

G. said recording medium passing between said rolls, means forelectrostatically squeezing said rolls so as to squeeze the recordingmedium between them,

H. and scraping means for compressing the surface of one of said rollsand scraping away the liquid at said roll.

13. Apparatus for printing on a flat flexible recording medium havingopposing faces comprising A. driving means for moving the medium along agiven path;

B. excitation means in the path of the recording medium forelectrostatically exciting the medium according to the image to beprinted;

C. developing means in the path of the recording medium for applying atoning liquid to one face of the excited recording medium;

D. fixing means in the path of the recording medium after saiddeveloping means for fixing the image and removing excess liquid fromthe recording medium;

E. said developing means including a. flow producing means for forming alaminar flow of toning liquid having a substantially flat surface, andb. guide means for longitudinally guiding the moving recording mediumalong a continuously curved course that intersects the moving liquid atonly one of its surfaces; and

F. said flow producing means including a. liquid slide means for formingan inclined upper surface spaced from said guide means so as to form agap between the curved path and the surface of said slide means, and

b. liquid moving means for passing the toning liquid over the surface ofsaid slide means so as to make the liquid flow gravitationally throughthe gap,

0. the size of said gap being sufficiently small so that when therecording medium is moved through the gap and the liquid flows throughthe gap the liquid forms a continuous meniscus at the medium in the gap;

G. said guide means including a conductive roller extending axiallyacross the longitudinal direction of movement of the medium; and

H. said liquid slide means being conductive and spaced from said rollerclosely enough to align the electrostatic field lines of the excitedmedium perpendicularly to the medium.

14. An apparatus for printing A. a continuous sheet of flexiblerecording medium composed of a conductively treated fiber base and asingle dielectric coating;

B. driving means for drawing the continuous flexible recording mediumand keeping it taut;

C. electrical excitation means for applying an electrostatic fieldacross the coating in the form of the i a eto rinted; D. iiet ielopi rfgmeans for applying a toning liquid having charged particles to thecoating on said recording medium;

E. drying means for removing excess liquid form the recording medium andallowing the charged particles to remain at the electrostaticallycharged portions of the recording medium;

F. said developing means including a. a roll in the path of saidrecording medium and co-operating with said driving means to apply anormal force against the base of said recording medium and about an arcof said roll;

b. liquid slide means forming an inclined plane parallel to the axis ofsaid roll, said axis being transverse to the direction of said movementof the recording medium;

0. pump means for pumping liquid to the top of said slide means;

d. nozzle means at the top of said slide means for directing said liquidover a wide path of the surface of said slide means;

e. said slide means being spaced from said roll so that said liquidencounters the coating of said recording medium as the recording mediummoves around said roll and forms a meniscus at said roll;

f. trough means for collecting the liquid flowing from said slide means;

g. a plurality of reservoirs holding a plurality of colors of toningliquid; and

h. valve means connecting said trough means to said reservoirs and saidreservoirs to said pump means so as selectively to select one of thereservoirs into the said pump means whereby one of several colors oftoning liquid may be used for toning said recording medium.

1. Apparatus for printing on a flat flexible recording medium havingopposing faces, comprising: A. driving means for moving the medium alonga given path; B. excitation means in the path of the recording mediumfor exciting the medium according to the image to be printed; C.developing means in the path of the recording medium for applying atoning liquid to one face of the excited recording medium; and D. fixingmeans in the path of the recording medium after said developing meansfor fixing the image and removing excess liquid from the recordingmedium; E. said developing means including: a. guide means forlongitudinally guiding the moving recording medium along a coNtinuouslycurved course such that the medium intersects the moving liquid at onlyone of its surfaces, and b. flow producing means for forming a laminarflow of toning liquid having a substantially flat surface, said flowproducing means including liquid slide means for forming an inclinedupper surface spaced from said guide means so as to form a gap betweenthe curved course and the slide means, and liquid moving means includinga flat nozzle means for spreading the liquid over the surface of theslide means so as to make the liquid flow gravitationally through thegap, the size of the gap being sufficiently small so that when therecording medium is moved through the gap the liquid forms a continuousmeniscus at the medium gap, the liquid encountering a continuous bandacross the medium at the meniscus.
 2. An apparatus as in claim 1 whereinA. said flow producing means includes control means for controlling theflow of toning liquid.
 3. An apparatus as in claim 1 wherein A. saidflow producing means includes control means responsive to said drivingmeans for controlling the flow of liquid over said surface.
 4. Anapparatus as in claim 1 wherein A. said liquid moving means includes aplurality of reservoirs of liquid of different colors and means forcontrolling the flow of said liquids so as to selectively pass one colorof liquid over said slide means.
 5. An apparatus as in claim 1 whereinA. said driving means move the medium longitudinally; B. said guidemeans include a roller extending transversely to the longitudinaldirection of the movement of the flexible recording medium andco-operating with said drive means to hole said recording medium tautover an arc of said roller and thereby to apply a normal forcecontinuously over said arc with said recording medium.
 6. An apparatusas in claim 1 wherein A. said driving means include a capstan fordriving said recording medium in a continuous longitudinal path throughsaid excitation means, said developing means and said drying means, andB. said driving means further include braking means for restrainingmovement of the recording medium so as to hold it taut.
 7. An apparatusas in claim 4 wherein A. said driving means move said recording mediumalong a continuous longitudinal path through said sensitizing means,said developing means, and said drying means and so the recording mediumextends simultaneously through said sensitizing means, said developingmeans and said drying means.
 8. An apparatus as in claim 1 wherein A.the gap between said guide means and said slide means is sufficientlysmall so the recording medium contacts the liquid and wherein B. saidgap is sufficiently large so as to keep the liquid in contact with onlyone side of the recording medium.
 9. An apparatus as in claim 1 whereinA. trough means for collecting liquid passing down said slide means, B.reservoir means for receiving liquid from said trough means, and C. pumpmeans for pumping the liquid to the upper surface of said slide means;D. whereby said liquid flows down said slide means.
 10. Apparatus forprinting on a flat flexible recording medium having opposing facescomprising; A. driving means for moving the medium along a given path;B. excitation means in the path of the recording medium for exciting themedium according to the image to be printed; C. developing means in thepath of the recording medium for applying a toning liquid to one face ofthe excited recording medium; D. fixing means in the path of therecording medium after said developing means for fixing the image andremoving excess liquid from the recording medium; E. said developingmeans including: a. flow producing means for forming a laminar flow oftoning liquid having a substantially flat surface, and b. guide meansfor longitudinally guiding the moving recording medium along acontinuously curvEd course that intersects the moving liquid at only oneof its surfaces; and F. said liquid removing means including a. firstand second engaging means for engaging opposite faces of the recordingmedium between them, b. at least one of said engaging means having anexternally curved surface so as to permit rolling engagement betweensaid engagement means when the recording medium is moved between them,c. said first engaging means having a compressible porous absorbentsurface and said second engaging means having a surface, d. pressuremeans for pressing the moving recording medium between said engagementmeans so that as said recording medium advances longitudinally saidengagement means squeeze continuously successive portions of therecording medium and the absorbent surface and then release therecording medium and the absorbent surface so that said absorbentsurface absorbs liquid on said recording medium, e. and compressionmeans for compressing the previously squeezed portion of the absorbentsurface and scraping it so as to remove the liquid therefrom and scrapeit away.
 11. An apparatus as in claim 10 wherein said engagement meanseach include rotatable roll means adapted to engage opposite faces ofthe same portion of the recording medium and squeeze the recordingmedium between them.
 12. Apparatus for printing on a flat flexiblerecording medium having opposing faces comprising; A. driving means formoving the medium along a given path; B. excitation means in the path ofthe recording medium for exciting the medium according to the image tobe printed; C. developing means in the path of the recording medium forapplying a toning liquid to one face of the excited recording medium; D.fixing means in the path of the recording medium after said developingmeans for fixing the image and removing excess liquid from the recordingmedium; E. said developing means including a. flow producing means forforming a laminar flow of toning liquid having a substantially flatsurface; and b. guide means for longitudinally guiding the movingrecording medium along a continuously curved course that intersects themoving liquid at only one of its surfaces; and F. said fixing meansincluding a pair of elastomeric rolls having absorptive surfaces forremoving excess liquid, G. said recording medium passing between saidrolls, means for squeezing said rolls so as to squeeze the recordingmedium between them, H. and scraping means for compressing the surfaceof one of said rolls and scraping away the liquid at said roll. 13.Apparatus for printing on a flat flexible recording medium havingopposing faces comprising A. driving means for moving the medium along agiven path; B. excitation means in the path of the recording medium forelectrostatically exciting the medium according to the image to beprinted; C. developing means in the path of the recording medium forapplying a toning liquid to one face of the excited recording medium; D.fixing means in the path of the recording medium after said developingmeans for fixing the image and removing excess liquid from the recordingmedium; E. said developing means including a. flow producing means forforming a laminar flow of toning liquid having a substantially flatsurface, and b. guide means for longitudinally guiding the movingrecording medium along a continuously curved course that intersects themoving liquid at only one of its surfaces; and F. said flow producingmeans including a. liquid slide means for forming an inclined uppersurface spaced from said guide means so as to form a gap between thecurved path and the surface of said slide means, and b. liquid movingmeans for passing the toning liquid over the surface of said slide meansso as to make the liquid flow gravitationally through the gap, c. thesize of said gap being sufficiently small so that when The recordingmedium is moved through the gap and the liquid flows through the gap theliquid forms a continuous meniscus at the medium in the gap; G. saidguide means including a conductive roller extending axially across thelongitudinal direction of movement of the medium; and H. said liquidslide means being conductive and spaced from said roller closely enoughto align the electrostatic field lines of the excited mediumperpendicularly to the medium.
 14. An apparatus for printing A. acontinuous sheet of flexible recording medium composed of a conductivelytreated fiber base and a single dielectric coating; B. driving means fordrawing the continuous flexible recording medium and keeping it taut; C.electrical excitation means for applying an electrostatic field acrossthe coating in the form of the image to be printed; D. developing meansfor applying a toning liquid having charged particles to the coating onsaid recording medium; E. drying means for removing excess liquid fromthe recording medium and allowing the charged particles to remain at theelectrostatically charged portions of the recording medium; F. saiddeveloping means including a. a roll in the path of said recordingmedium and co-operating with said driving means to apply a normal forceagainst the base of said recording medium and about an arc of said roll;b. liquid slide means forming an inclined plane parallel to the axis ofsaid roll, said axis being transverse to the direction of said movementof the recording medium; c. pump means for pumping liquid to the top ofsaid slide means; d. nozzle means at the top of said slide means fordirecting said liquid over a wide path of the surface of said slidemeans; e. said slide means being spaced from said roll so that saidliquid encounters the coating of said recording medium as the recordingmedium moves around said roll and forms a meniscus at said roll; f.trough means for collecting the liquid flowing from said slide means; g.a plurality of reservoirs holding a plurality of colors of toningliquid; and h. valve means connecting said trough means to saidreservoirs and said reservoirs to said pump means so as selectively toselect one of the reservoirs into the said pump means whereby one ofseveral colors of toning liquid may be used for toning said recordingmedium.